One of the most complex and difficult problems that has plagued the medical profession and pharmaceutical industry for decades is the problem of achieving a therapeutic concentration of a drug locally at a target site within the body without producing unwanted systemic side effects. Parenteral or oral therapy of substances directed at treating disease in a particular internal organ must often be given in amounts dependant upon achieving critical systemic blood levels that can produce devastating side effects at other areas in the body. A prime example of a situation where local therapy is needed with drugs that also produce unwanted systemic side effects is the prevention of complications following the placement of a cardiovascular prosthetic device such as a prosthetic vascular graft or a patch used to repair a damaged vessel.
Graft failure is often associated with the inherent thrombogenicity of the blood contacting surface of the prosthetic device and with the body's own repair mechanisms which can lead to progressive stenotic occlusion due to neointimal fibrosis and hyperplasia. Systemic therapy aimed at preventing coagulation and thrombosis locally at the graft site is often complicated by bleeding at other sites. Likewise, systemic treatment with growth mediators or chemotherapeutic agents can produce a hyperplastic or hypoplastic response in tissue not specifically targeted. Similarly, administration of vasodilators can produce systemic hypotension.
There have been many attempts to render vascular grafts less thrombogenic, e.g., by coating the luminal surface of the graft with non-thrombogenic polymers (U.S. Pat. No. 4,687,482), cells (U.S. Pat. No. 5,037,378) or with anticoagulant drugs in a polymer coating (PCT Application WO 91/12279). Although these attempts have improved the success associated with graft placement, complications with clotting, thrombosis, and restenosis, especially that seen due to fibroplasia and smooth muscle proliferation, still abound. Therefore, there exists a need in the art for a means and a method of providing local therapy which can sustain high local concentrations of therapeutic drugs at the site of vessel repair thereby preventing complications associated with placement of the prosthetic graft without producing unwanted systemic side effects.